Hydroxyalkyl starches comprise a commercially important class of chemicals. Such hydroxyalkyl starches are useful in a variety of commercial and industrial applications and for a variety of purposes, such as for enhancing freeze/thaw and cold storage stability, for lowering the gelatinization temperature of mixtures such as in application for meat products, for increasing the viscosity and clarity of gels and pastes, for reducing gel syneresis, and for many other uses. HydroxyaLkyl starches are particularly suitable as food thickeners, and such starches are often used commercially in such applications.
The prior art has provided a number of methods for preparing hydroxyalkyl starches. Conventionally, hydroxyalkyl starches are prepared by reacting starch with an alkylene oxide, generally in the presence of a suitable catalyst, such as an alkali metal hydroxide catalyst. Commonly, such reactions take place in a aqueous slurry or suspension, from which further modification of the starch can be conveniently accomplished and from which the hydroxyalkyl starch can readily be removed and washed. Such conventional processes are further discussed in Whistler et al., Starch Chemistry and Technology, 2d. ed. 1984.
The conventional method of hydroxyalkylating starch in aqueous slurry is limited in some respects. The extent of substitution may be measured by the molar substitution ("MS") of the starch, which expresses the number of substituted monomeric units per anhydroglucose unit. It is commonly understood that, when a starch is hydroxyalkylated conventionally in an aqueous suspension to an MS greater than about 0.1, the integrity of the starch granules will become compromised, and the starch will become gelatinized. Starch gelatinization results from the swelling and degradation of the starch granules in the aqueous slurry. If such swelling occurs to an extent that the starch gelatinizes, such gelatinization will lead to substantial difficulties in further processing the finished hydroxylated starch. In particular, gelatinization of the starch will render it difficult to remove soluble reaction by-products, which otherwise could be removed merely by washing. Thus, in commercial practice, the conventional method is limited to the preparation of hydroxyalkyl starches that have an MS of about 0.1 or less.
Besides the conventional method, other methods for preparing hydroxylated starches are also known in the art. In accordance with such methods, hydroxylated starches that have an MS of greater than about 0.1 can be provided. Typically, such methods involve hydroxyalkylating the starch under low-moisture conditions, with the reaction taking place in the dry state or in a non-aqueous medium, such as in an alcoholic medium. Details concerning such methods can be found in one or more of U.S. Pat. Nos. 2,516,632; 2,516,634; and 2,845,417. However, while such methods are useful in obtaining high MS hydroxyalkyl starches, such methods suffer from a number of drawbacks that limit their practicality in commercial applications. Dry state processes are expensive because of the need to protect against explosive hazards presented by starch dust, and because of the special equipment needed. Similarly, reactions in non-aqueous solvents also entail significant additional process costs. The use of such solvents can also present fire and explosion hazards, especially when a lower aliphatic alcohol is used as the reaction medium.
It is a general object of the present invention to provide a method for preparing a hydroxyalkyl starch. In accordance with the preferred embodiments of the present invention, it is another general object to provide a method for preparing a hydroxyalkyl starch in an aqueous medium to provide a hydroxyalkyl starch having an MS greater than about 0.1, whereby the starch remains in granular form after hydroxyalkylation and does not gelatinize.